1. Field of the Invention
The present invention relates to the field of microelectronics and in particular to digital circuits. Still more particularly, the present invention relates to output drivers in digital circuits.
2. Description of the Prior Art
In a digital circuit system, a common path is often used to share and transfer data between various circuits and devices in the system. A small set of shared lines, a bus, may be used to provide the common path. In the designing of digital circuit systems, some devices may have an insufficient amount of maximum output current to drive all of the lines connected to the devices. As a result, a current amplifier, called a driver or a buffer, is used to provide the needed currents.
Latches are often used to control output drivers in digital circuit systems. The state of these latches is often critical during the power up phase of a digital circuit system. For example, in an output driver circuit having a pull up and pull down transistor and having two latches controlling the pull up and pull down transistors, forming the output driver, the individual latches may power up in states that turn on both transistors at the same time. Such a condition is known as a "crowbar" condition. Frequently, parts that have differential data bussed all the way to the output can see such effects.
In some cases, a power-up reset circuit is used to force the state of the latches to some preset state to avoid a crowbar condition. Such a solution is not always sufficient. For example, sometimes the state of the differential buses may not be differentiated, leading to a crowbar condition during non-power-up conditions. Also, a glitch in device operation or external conditions such as a power supply may cause non-differentiation during a non-power-up condition.
In other cases, during testing of the device, a defective memory cell may be accessed, rendering the differential buses non-differentiated and creating a crowbar condition at the outputs. In such a situation, the bad memory cell may be replaced by redundant memory cells, but the testing of the bad memory cell may have fused the power supplies or in some other way rendered the device unusable.
Therefore, it would be desirable to have an apparatus to prevent a crowbar condition from occurring in a output driver controlled by two or more latches.